Recent improvements in the power handling capabilities and energy densities for electronic switches have facilitated the development of direct-current (DC) power distribution systems. FIG. 1 is a schematic of an example DC distribution power system, where the DC distribution network is connected to a transmission system via appropriate power transformation and conversion interfaces. There are alternating-current (AC) generation, DC distributed generation (DG), energy storage (ES), and load subsystems connected to each bus by converters or lines. The various DC buses, identified in FIG. 1 as DC1, DC2, etc., are connected to one another by branches (overhead lines or cables) or converters.
Shown in FIG. 1 are a number of converters, including AC/DC converters 110 and DC/DC converters 120. For the purposes of the present disclosure, converters of either of these types may be referred to herein as simply DC converters, meaning that the converters have at least DC power interface. It will be appreciated that these converters may be uni-directional or bi-directional with respect to power flow, depending on their specific applications, and may be of any of several known designs.
As seen in FIG. 1, a group of equipment can be connected to any particular DC bus. Systems are typically designed so that power supply to any particular group of loads can be provided from one or several alternative DC buses, to meet reliability requirements for the system. The system includes several protective devices, such as DC switches 130. Typically at least one protective device is installed next to each DC bus to isolate faults on DC buses or downstream branches.
DC distribution systems like that shown in FIG. 1 and its variations may be used in DC distribution networks, DC industrial systems, DC renewable energy collection systems, DC shipboard power systems, DC data centers, DC building systems, etc. A DC distribution system may be coupled to one or more AC transmission systems and/or AC distribution systems.
Electric power utilities in the US and around the world are currently in the process of upgrading their AC distribution systems to simplify and automate system operation by implementing enhanced monitoring, distribution automation and control solutions. The ultimate goal from a distribution system operations standpoint, as stated by many utilities in their roadmaps to the so-called Smart Grid, is to achieve smart, self-healing grids. These grids should be capable of automatic isolation of permanent faults and automatic system reconfiguration, to quickly restore power to as many customers as possible by switching affected customers over to alternative sources of power in the event of an interruption.
This goal can be achieved within the distribution management system (DMS) framework by adding various smart sensors, integrating sensor and meter data into decision making process, and using advanced hybrid (wired/wireless) communications infrastructure, to implement automatic fault location, isolation and load restoration schemes.
Several problems arise when trying to apply these automatic fault isolation and recovery techniques to DC distribution systems. Due to small resistances and a lack of inductance in these systems, when a DC fault occurs, the rate of rise of DC fault current is quite fast, and the peak fault current is very high. Normally, DC fault current can reach its peak current in very short time. The fast rate of rise creates difficulties for fault isolation, and high DC fault current may damage equipment in the protected DC distribution system.
In a converter-based DC distribution system, large fault currents cannot be allowed for very long, due to operation limits of equipment or devices. For example, the maximum current that can be allowed to flow through power electronic switches is limited by its Safe Operating Area (SOA). FIG. 2 shows a typical SOA of a power electronic switch, as might be found in its device datasheet. Any operation within the boundaries indicated the by bold lines in the figure as 1) current boundary; 2) thermal boundary; 3) secondary breakdown boundary; 4) voltage boundary), is safe and allowed. All four boundaries, except for the secondary breakdown boundary, exist for all power electronic switches.
Improved techniques for fault isolation and recovery, specific to the problems that arise in DC distribution system, are needed.